1. Field of the Invention
This invention relates to a catalyst deterioration-determining system for internal combustion engines using a compressed natural gas, which determines deterioration of a catalyst arranged in the exhaust system of the engine, for purifying exhaust gases emitted from the engine.
2. Prior Art
A catalyst deterioration-determining system is known, e.g. from Japanese Laid-Open Patent Publication (Kokai) No. 5-248227, which determines deterioration of a catalyst provided in the exhaust passage of an internal combustion engine, which controls the air-fuel ratio of an air-fuel mixture supplied to the engine in a feedback manner responsive to an output from an oxygen concentration sensor arranged in the exhaust passage at a location downstream of the catalyst, determines that the catalyst is deteriorated, when a period of change of the output of the oxygen concentration sensor is shorter than a predetermined reference value which is set based upon the temperature of the catalyst.
The known system, however, suffers from the following problem if it is employed in an internal combustion engine using a compressed natural gas (hereinafter referred to as "CNG") instead of gasoline.
That is, CNG contains methane as a main ingredient. The methane cannot be easily oxidized by the catalyst so that part of unburnt methane in exhaust gases emitted from the engine flows downstream of the catalyst. In such an event, if the electrode temperature of the oxygen concentration sensor is high, reaction of the unburnt methane with oxygen is promoted at or in the vicinity of the electrodes of the sensor, whereby the unburnt methane burns. This causes consumption of oxygen in the vicinity of the electrodes so that the partial pressure of oxygen in the vicinity of the electrodes changes in such a direction that the period of change of the output of the oxygen concentration sensor becomes shorter. FIG. 1 shows an output characteristic of the oxygen concentration sensor obtained in the above event. In the figure, T1, T2 and T3 represent values of the temperature TSO2 of the sensor element and are in the relationship of T1&lt;T2&lt;T3. As shown in the figure, as the sensor element temperature TSO2 rises,the period of change TPRD of the sensor output becomes shorter.
In the known catalyst deterioration-determining system, the predetermined reference value with which the period of change of the output of the oxygen concentration sensor is compared is set based upon the temperature of the catalyst, as mentioned above. However, the setting of the predetermined reference value does not take into consideration the temperature of the oxygen concentration sensor. Consequently, when the above manner of determining deterioration of the catalyst is applied to an internal combustion engine using CNG as fuel, there is a possibility that the catalyst is determined to be deteriorated even when the catalyst is not actually deteriorated.